It makes no difference if other scripts set/use _ before or after this.
(lodash remembers the last setting of _ and restores it on the _.noConflict() call.)

But if scripts before this are async there is always a possibility that they are allowed to execute between these two scripts. You would have to either use AMD or combine the manual setting into the same script as lodash to avoid races with async scripts.

Email codedump link for How to correctly include lodash/underscore on website with conflicting libs?